Electroabsorption modulator with two sections

ABSTRACT

The invention relates to an electroabsorption modulator (EAM) ( 300 ), comprising a first EAM section ( 302 ) optically coupled to a second EAM section ( 304 ), a transition wavelength in the electroabsorption (EA) spectrum of the first EAM section ( 302 ), at which absorption changes substantially, being different to a transition wavelength in the EA spectrum of the second EAM section ( 304 ). The EAM ( 300 ) compensates for chirp (which is caused by intrinsic absorption effects) by driving the two sections with a signal ( 312 ) generated from a common modulating signal ( 314 ). The driving signal ( 312 ) at the first EAM section ( 302 ) being preferably in anti-phase with the driving signal ( 312 ) at the second EAM section ( 304 ).

FIELD OF THE INVENTION

The present invention relates to a high speed modulating device for usein optical communication systems, and in particular a chirp-compensatedelectroabsorption modulator.

BACKGROUND TO THE INVENTION

The continuing growth in the volume of information to be transported byoptical communication systems is placing increasing demands on the speedand bandwidth of these systems. In order to satisfy this need opticalcomponents capable of dealing with the higher data rates and broaderbandwidth are required. This includes the optical fibre used as thetransport medium and components such as the optical modulator, used toencode data onto an optical signal.

A number of techniques have been employed to produce fast opticalmodulators, including a Mach-Zehnder Interferometer (MZI) typearrangement. However, the electroabsorption modulator (EAM) has emergedas a alternative, and often preferred choice, due to its low voltagerequirements and relatively compact physical dimensions.Electroabsorption modulators have already been implemented in 10 Gb/soptical fibre based communication systems. Like the MZI modulator, theEAM usually comprises a waveguide section for optical confinement, inorder to be compatible with fibre systems. The EAM can be integratedwith a laser source in a single module or may be fabricated as a standalone device.

The EAM operates via an electric field induced change in the absorptionspectrum, the so-called electroabsorption (EA) effect A number of veryfast physical mechanisms may be involved in this spectral shift,including the linear and quadratic Stark effect. In order to enhance theperformance of such devices, multiple quantum well (MQW) structures havebeen included, thereby taking advantage of the quantum-confined Stark(QCS) effect. As a consequence, a small applied electric field caninduce a large change in absorption at a particular wavelength. Forexample, the application of a reverse bias voltage of a few volts to aMQW based EAM results in a bandgap shift to longer wavelengths and adevice extinction ratio as high as 20 dB.

Although the underlying mechanism is very fast, the speed of response ofa conventional lumped-element EAM to an electrical driving signal islimited by intrinsic and stray capacitance, thereby limiting the usefulmodulation bandwidth. One approach to tackling this problem is the useof a shorter EAM waveguide to reduce device capacitance. However, thisapproach tends to compromise modulation efficiency and extinction ratio.FIG. 1 shows an example of one of the more successful lumped EAMs 100,where a short MQW based EAM 102 is integrated with transparent input 104and output 106 waveguides and fabricated on an InP:Fe substrate 110 toreduce the stray capacitance.

The EAM 102 illustrated has a length of 75 μm, while the wavelength, λ,of the optical input 108 is 1553 nm. A biasing voltage is applied acrossthe EAM 102 between a p-electrode 112 and an n-electrode 114. Theoptical output 116 exhibits an extinction effect corresponding to theelectroabsorption in the EAM 102.

A more successful technique for increasing the useful modulationbandwidth of an EAM makes use of the travelling wave effect that ariseswhen a driving microwave signal copropagates alongside an optical signalconfined within the EA region of an EAM. The so-called travelling-waveEAM (TW-EAM) has been investigated experimentally and shown to exhibitsuperior performance to the lumped element EAM. In a typical TW-EAM, anelectrode structure 202 is employed that provides a transmission linefor the driving microwave signal to propagate alongside the opticalsignal. This transmission line ensures good overlap of the modulationfield with the EA region, permitting high speed operation with goodmodulation characteristics. An example of a known TW-EAM 200 is shown inFIG. 2.

The electrode structure 202 is provided as a metal on a SI—InPsubstrate. An optical waveguide structure 204 is formed over anunderlying n-layer 206, and is optically aligned with a transmissionline formed by the electrode structure 202. Optical input arriving alongthe optical waveguide 204 enters the transmission line by way of apolyimide bridge 208.

A major challenge associated with the TW-EAM is the accurate velocitymatching of the optical and microwave signals over a broad bandwidth.The fundamental sources of temporal mismatch of the two signals are therespective lengths of the waveguide and transmission line and also theresistance to signal propagation, as measured by the modal refractiveindex and line impedance for the optical and microwave signals,respectively. One approach to achieving velocity matching is the use ofa TW-EAM comprising several discrete EAM regions, such that atransmission line can be fabricated which overlaps these regions butwhere the microwave signal follows a longer path than the opticalsignal, thereby compensating for the faster speed of propagation.

However, even with adequate velocity matching, a further problemassociated with the TW-EAM is the broadening of an optical signal'sbandwidth during propagation through the TW-EAM. The problem can alsooccur for the lumped EAM, but is of less importance at the modulationfrequency typically used. The problem manifests itself as an apparentdifference in the speed of propagation of the different wavelengthcomponents contained within the encoded optical signal, a characteristicknown as chirp. The origin of this problem is an unwanted phasemodulation of the optical signal that can accompany the intendedamplitude modulation. When a driving electric field is applied to anEAM, the resulting change in the active material's bandgap leads to achange in absorption, which effects the amplitude modulation of theoptical signal. However, associated with the change in the materialbandgap is a change in the material refractive index, which results inthe unwanted phase modulation of the optical signal. As the broadenedsignal leaves the EAM and enters an optical communications network, theproblem is compounded still further by the inherent dispersion exhibitedby optical fibre in the network. The wavelength components in thebroadened spectrum of the optical signal each experience a differentrefractive index in the fibre leading to even further broadening of thespectrum.

As a result of signal chirp, the integrity of data encoded on an opticalsignal may be compromised. For example, data encoded digitally may berepresented by a series of temporal spikes which, in the presence ofchirp, may develop a pedestal that blurs the distinction betweenindividual bits of data. This can be particularly troublesome when thedata stream comprises several signals from different sources that havebeen multiplexed by an optical time division multiplexing (OTDM) system.Similarly, a signal with a broadened bandwidth may result in dataintegrity being compromised, if multiplexed with other signals in awavelength division multiplexing (WDM) system.

Thus, the elimination or negation of chirp in EA modulators, especiallythe TW-EAM, is of key importance for high bit-rate, long haul systems,particularly for the 40 GHz and above required by next generationoptical communication systems.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an electroabsorptionmodulator (EAM) comprises a first EAM section optically coupled to asecond EAM section, a transition wavelength in the electroabsorption(EA) spectrum of the first EAM section, at which absorption changessubstantially, being different to a transition wavelength in the EAspectrum of the second EAM section, wherein the first EAM section andsecond EAM section are driven by separate radio frequency (RF) signals.

Preferably, the separate radio frequency (RF) signals are generated independence on a common modulating RF signal and have a phase differencebetween them. Preferably, the phase difference is substantially 180°such that the first EAM section and second EAM section are driven inanti-phase in dependence on the common modulating RF signal.

In this preferred embodiment, the sections are optically coupled anddriven in anti-phase in dependence on a common modulating RF signal.Alternatively, the separate radio frequency (RF) signals could begenerated in dependence on two respective independent modulating RFsignals and have a phase difference between them. In which case, thephase difference would advantageously be substantially 180° such thatthe first EAM section and second EAM section are driven in anti-phase independence on the two respective independent modulating RF signals.

In either case, the amplitudes of the RF signals driving each of thefirst and second EAM sections are preferably controlled independently.An optical signal passing through the EAM, with a wavelength similar tothe EA transition wavelength of one of the first and second EAMsections, will be amplitude modulated by that EAM. At the same time, theother of the first and second EAM sections will preferably besubstantially transparent at the wavelength of the optical signal, bothin the presence or absence of the RF signal. Thus, no amplitudemodulation will be imposed on the optical signal by this EAM section. Inthis way, the amplitude of the optical signal may be modulated by one ofthe first and second EAM sections whilst being substantially unaffectedby the other EAM section.

However, due to the dispersion characteristics of an EAM, an opticalsignal passing through an EAM will experience phase modulation, leadingto signal chirp and associated spectral broadening, even when thewavelength of the optical signal is different to the EA transitionwavelength of the EAM. The amount of phase modulation (chirp) impartedto the signal will depend not only on the dispersion characteristics ofthe EAM material but also on the EAM length and/or the strength of thedriving RF signal. By driving two EAM sections in anti-phase, inaccordance with the present invention, an amplitude modulated opticalsignal may be generated with a controlled amount of chirp, includingpositive, negative or zero chirp.

Therefore, in an EAM according to the present invention, any unwantedphase modulation accompanying the amplitude modulation of an opticalsignal by one of a first and second EAM sections, may be wholly orpartially compensated for by phase modulation applied, in anti-phase, bythe other of the first and second EAM sections. If the length anddispersion characteristics of the first and second EAM sections aresimilar, the resultant amount of chirp will be small and may befine-tuned by the relative strength of the RF driving signals applied tothe two EAM sections.

Preferably, a fast optical chirp detection system is employed to providean error signal for feedback to the driving circuitry for the RFsignals, thereby providing for dynamic correction of signal chirp in theEAM. In the absence of such a system a predetermined signal is appliedto the two EAM sections, based on prior testing of the electroabsorptivedispersion characteristics of the two EAM sections.

Generating an amplitude modulated optical signal with a predefinedamount of positive or negative chirp may be desirable in the dispersionmanagement of optical networks. An example would be the“pre-compensation” of group velocity dispersion or non-linear phasemodulation that may affect the optical signal during propagation throughother components in the optical network.

Although the present invention may comprise a discrete first and secondEAM section, it is preferred that the invention comprises a first EAMsection integrated with a second EAM section. Preferably, the first andsecond EAM sections are monolithically integrated on a commonsemiconductor substrate. Preferably, the substrate comprises an indiumphosphide (InP) based material, as this allows integration with otherdevices, such as a laser diode.

There are a range of materials and mechanisms available for thefabrication of an EAM.

Preferably, the first and second EAM sections comprise a multiplequantum well (MQW) structure. Preferably, over the desired range ofoperational optical wavelengths, the MQW structure is optimized to beelectroabsorbing in one EAM section and transparent in the other EAMsection.

Transparency may be achieved by ensuring the EAM material exhibits alarge bandgap. Appropriate tailoring of this bandgap may be achieved byselective epitaxial growth or by use of a quantum well intermixing (QWI)process. QWI may also be used to smooth the transition region betweenthe electroabsorbing EAM section and the transparent EAM section in amonolithically integrated version of the present invention.

The EAM according to the present invention may be of the lumped type ormay be of the travelling wave (TW) type. In each case it is preferredthat the EAM comprises a first EAM section and a second EAM section ofthe corresponding type.

Typically a lumped EAM is used at a lower modulation frequency of 10Gbit/s, where signal chirp due to optical phase modulation is not asignificant problem. Signal chirp is, however, a problem at frequenciesof 40 Gbit/s and above where the travelling-wave EAM (TW-EAM) iscommonly used.

The present invention thus provides an EAM, preferably a TW-EAM, forhigh-speed, broad-band amplitude modulation of an optical carrier signalwith zero signal chirp, suitable for long haul, operation where opticalfibre dispersion may become significant. Furthermore, a controlledamount of positive or negative chirp may be imparted to the modulatedsignal for the pre-compensation of such dispersion.

According to another aspect of the present invention, an optical devicefor optical time division multiplexing or demultiplexing comprises anEAM in accordance with the one aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present invention will now be described in detail withreference to the accompanying drawings, in which:

FIG. 1 is a schematic of a known EAM;

FIG. 2 is a top view of a known TW-EAM;

FIG. 3 shows a schematic of a TW-EAM with chirp-compensation, inaccordance with the present invention; and,

FIG. 4 shows the electroabsorption spectrum for the two regions, parts Aand B, of the TW-EAM shown in FIG. 4.

DETAILED DESCRIPTION

The present invention provides an apparatus for modulating an opticalcarrier signal, with a controlled amount of signal chirp, by means oftwo optically coupled EAM sections, one of which is electroabsorbing andthe other of which is optically transparent at the optical signalwavelength, the two EAM sections being driven in anti-phase independence on a common modulating RF signal.

Typically, an EAM is optically transparent at a wavelength longer thanits EA transition wavelength. Typically, therefore, the EA transitionwavelength of the electroabsorbing EAM section will be longer than thatof the transparent EAM section. By applying a suitable driving RF signalto the electroabsorbing EAM section, an optical carrier signal passingthrough the EAM may be amplitude modulated. Any unwanted phasemodulation accompanying the amplitude modulation may be compensated forby phase modulation applied in anti-phase to the optical signal by meansof the transparent EAM section.

FIG. 3 shows a schematic of an EAM 300 in accordance with the presentinvention. The EAM 300 is of the travelling-wave type and thereforecomprises two TW-EAM sections, monolithically integrated on a commonsubstrate. The electroabsorbing 302 and the transparent 304 parts of theTW-EAM are denoted as Part A and Part B, respectively. The corecomponents of each of the TW-EAM sections are an optical waveguide 322for light confinement, including a MQW structure, and a striptransmission line 320 located above the optical waveguide 322. Bothtransmission lines 320 are terminated by a suitable resistive load 306,308 to avoid significant reflection of the driving RF signal, which maylead to timing jitter and distortion of the modulated optical signal.

As for the known TW-EAM 200 of FIG. 2, the electrodes of the EAM 300 ofFIG. 3 may be provided as a metal layer over a SI-lnP substrate, thislayer being electrically grounded except for the transmission lines 320.

The MQW structure in Part A is optimized to be electroabsorbing at thedesired operational optical wavelength. The optical transparency of PartB is achieved by increasing the bandgap of the MQW structure, either byselective epitaxial growth or by post growth modification using a QWIprocess. The bandgap in Part B can be engineered such that the EAtransition wavelength is sufficiently below the operational wavelengthfor optical transparency, but close enough that a sufficient amount ofdispersion can be experienced by an optical signal to facilitate chirpcompensation. To smooth the transition at the interface between Parts Aand B, and reduce optical losses, a QWI process may be applied to theMQW structure in the region of the interface.

A differential amplifier 310 is employed to drive the two TW-EAMsections 302, 304 in anti-phase with a common modulating RF signal 312.A single modulating signal 314 is fed to the differential amplifier 310,which produces two copies of the original signal with a 180° phase shiftbetween them, but which are otherwise time-synchronized. A suitable timedelay may be applied to the signal for driving the transparent TW-EAMsection 304 so that the modulated optical and RF signals areappropriately time-synchronized in the TW-EAM. Furthermore, as shown inFIG. 3, a variable attenuator 316 is employed to adjust the strength ofthe RF signal applied to Part B, and thereby fine-tune the amount ofcompensating phase modulation.

While FIG. 3 illustrates an embodiment where the sections are driven inanti-phase in dependence on a common modulating RF signal, the sectionscould alternatively be driven in dependence on two independentmodulating RF signals (not shown).

FIG. 4 shows a typical electroabsorption spectrum for Parts A and B ofthe device depicted in FIG. 3. The desired operational wavelength iswithin the electroabsorption transition region of Part A. Therefore, asshown, in the absence of an applied electric field (solid line 402) theEA region of Part A is substantially transparent When an RF electricfield is applied (broken line 404), the EA region of Part A becomesabsorbing, thus permitting amplitude modulation of an optical carriersignal.

In addition to the change in the absorption spectrum, when an RF signalis applied, there is an associated change in the dispersioncharacteristics or refractive index spectrum (not shown here). It isthis change which gives rise to the unwanted phase modulation of theoptical carrier signal and hence signal chirp. A similar change in thedispersion characteristics of the EA region of Part B occurs when the RFsignal is applied, a feature which can be used to compensate for thephase modulation acquired in Part A. As can be seen from FIG. 4, the EAtransition wavelength of Part B is significantly shorter (bluer) thanthe desired operational wavelength λ_(op). Therefore, even in thepresence of an applied RF electric field 408, Part B remainssubstantially transparent and contributes no amplitude modulation to theoptical signal.

If the length and dispersion characteristics of parts A and B are verysimilar, accurate compensation, and hence zero signal chirp, can beobtained by driving the two regions with an RF signal of similarstrength, but in anti-phase. In the example of FIG. 3, a weaker signalis required to drive Part B to achieve chirp compensation and isachieved by appropriate adjustment of the variable attenuator 316. Bysetting the variable attenuator 316 to values above and below thatrequired for compensation, a controlled amount of positive or negativechirp may be obtained. If a stronger signal is required to drive Part Bto compensation or beyond, the variable attenuator 316 can be replacedby an amplifier (not shown). However, it is preferable to supply astronger RF signal to the differential amplifier 310 and add a variableattenuator 316 to the circuitry connecting to the transmission line 320of Part A.

The ability to generate an amplitude modulated optical signal with asmall amount of controlled chirp may be helpful in the dispersionmanagement of optical networks. In optical communication systems wherehigh data rate optical signals are transmitted over long haul distances,even small amounts of dispersion in the optical fibre can have asignificant cumulative effect Therefore, a modulated optical signal“pre-chirped” with the correct amount of oppositely signed chirp couldbe used to negate the effects of fibre dispersion in long haultransmission.

In the example of FIG. 3, the amplitude modulating TW-EAM section 302precedes the transparent TW-EAM section 304, in terms of the directionof light propagation. However, the order of the two TW-EAM sectionscould be reversed. Indeed the overall EAM could comprise an amplitudemodulating EAM section optically coupled to two transparent EAM sectionslocated either side of the amplitude modulating EAM section, each drivenindependently by a common RF signal.

Typically, in all these devices, a fixed predetermined signal strength(voltage) is applied to the EAM sections based on prior testing andknowledge of the dispersion characteristics of the EAM sections, inorder to obtain the desired signal chirp. However, if a very fastoptical detection system is able to monitor the amount of signal chirp,this can provide an error signal. Such a signal, when fed back to the RFdriving circuitry, can provide for dynamic stabilization of the totalsignal chirp, including precise compensation or zero chirp.

Thus, the present invention provides an EAM suitable for high-speed,broad-band amplitude modulation of an optical carrier signal with zerosignal chirp. A travelling-wave embodiment of the device is particularlysuitable for high bit-rate, long haul operation where optical fibredispersion may become significant. Furthermore, a controlled amount ofpositive or negative chirp may be imparted to the modulated signal forthe pre-compensation of such dispersion.

1. An electroabsorption modulator (EAM), comprising a first EAM sectionoptically coupled to a second EAM section, a transition wavelength inthe electroabsorption (EA) spectrum of the first EAM section, at whichabsorption changes substantially, being different to a transitionwavelength in the EA spectrum of the second EAM section, wherein thefirst EAM section and second EAM section are driven by separate radiofrequency (RF) signals.
 2. An EAM according to claim 1, wherein theseparate radio frequency (RF) signals are generated in dependence on acommon modulating RF signal and have a phase difference between them. 3.An EAM according to claim 2, wherein the phase difference issubstantially 180° such that the first EAM section and second EAMsection are driven in anti-phase in dependence on the common modulatingRF signal.
 4. An EAM according to claim 1, wherein the separate radiofrequency (RF) signals are generated in dependence on two respectiveindependent modulating RF signals and have a phase difference betweenthem.
 5. An EAM according to claim 4, wherein the phase difference issubstantially 180° such that the first EAM section and second EAMsection are driven in anti-phase in dependence on the respectiveindependent modulating RF signals.
 6. An EAM according to claim 2,wherein the amplitudes of the RF signals driving each of the first EAMand second EAM sections are controlled independently.
 7. An EAMaccording to claim 2, wherein one of the first EAM and second EAMsections is substantially transparent at a wavelength at which the otherof the first EAM and second EAM sections is substantiallyelectroabsorptive.
 8. An EAM according to claim 2, wherein the sign andmagnitude of phase modulation imparted to an optical signal propagatingthrough the EAM is determined by the lengths of the first EAM and secondEAM sections.
 9. An EAM according to claim 8, wherein the sign andmagnitude of the phase modulation imparted to the optical signal isfurther determined by the amplitudes of the RF signals driving each ofthe first EAM and second EAM sections.
 10. An EAM according to claim 9,wherein the amplitudes of the RF signals driving the first EAM andsecond EAM sections are determined in dependence on an optical devicemonitoring the phase modulation imparted to the optical signal.
 11. AnEAM according to claim 2, wherein the first EAM and second EAM sectionsare integrated on a common substrate.
 12. An EAM according to claim 2,wherein at least one of the first EAM and second EAM sections comprisesa multiple quantum well (MQW) structure.
 13. An EAM according to claim2, wherein each of the first EAM and second EAM sections comprise atraveling wave EAM (TW-EAM).
 14. An optical device for optical timedivision multiplexing comprising an EAM according to claim 2.